1. Field of the Invention
This invention relates to a transmission system and, more particularly, to a transmission system for controlling the transmission of a multiplexed signal.
2. Description of the Related Art
SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy), being the nucleus of multiplexing technologies, defines communication procedures for multiplexing various communication services effectively. The standardization of SONET/SDH is stimulating its development.
In addition, in recent years data traffic has increased especially on the Internet and various communication services are needed. Therefore, it is hoped that SONET/SDH transmission systems with larger capacity will be introduced into the current backbone networks.
FIG. 20 is a view showing the structure of the format of a synchronous transport signal-1 (STS-1). An STS-1 (51.84 Mbps) is a frame being the standard unit for SONET. On the other hand, a synchronous transport module-1 (STM-1: 155.52 Mbps) is a frame being the standard unit for SDH.
The format of an STS-1 frame consists of 9 (rows)×90 (bytes). In FIG. 20, the leftmost three bytes are used as an overhead (OH) area. The remaining eighty-seven bytes less a path overhead (POH) area are a payload where actual user data will be inserted.
With SONET/SDH, multiplexing will be controlled by the use of the standardized multiplexing unit called a virtual container (VC). Multiplexing by a VC is always performed by the byte. VC's are concatenated to generate a concatenation signal. With SONET, for example, a concatenation signal with a capacity of N times the capacity of a VC is symbolized as STS-Nc.
FIG. 21 is a view showing the structure of the format of an STS-12c. An STS-12c frame is represented by a two-dimensional byte array consisting of 9 (rows)×1080 (columns). The leading 9 (rows)×36 (columns) are an OH area and the subsequent 9 (rows)×1044 (columns) are a payload where multiplexed information is stored. Four STS-3c sets, CH1 through CH4, are multiplexed in this payload portion.
Byte H1 is located in column 1 to column 12 of a pointer in row 4 and byte H2 is located in column 13 to column 24 of the pointer in row 4. Bits 7 and 8 of byte H1 and 8 bits of byte H2 form a 10-bit pointer.
Bits 1 through 4 (4 bits) of byte H1 are a new data flag (NDF). With NDF, the value of a 10-bit pointer will be changed according to a change in the payload. For example, when there is no need to change the value of this pointer, NDF is represented by the code “0110.” When the value of this pointer is changed, NDF is represented by the reverse code “1001.”
However, an NDF for a subsidiary channel is “1001” (constant). A subsidiary channel of a concatenation signal is indicated by a combination of this NDF and a pointer value.
Usually the value of a 10-bit pointer for a leading channel is a number between 0 and 782. On the other hand, the bits of a 10-bit pointer for a subsidiary channel of a concatenation signal are all “1.” A combination of the NDF (“1001”) and the 10-bit pointer (“1111111111”) indicates that it is a subsidiary channel. For example, an STS-12c consists of four channels. Therefore, the value of a 10-bit pointer for channel 1 is a number between 0 and 782. Channels 2, 3, and 4 are subsidiary to channel 1, so the bits of 10-bit pointers for them are all “1.”
Generating and transmitting such a concatenation signal enables large-capacity transmission which cannot be realized by a single container.
The provision of new services, however, will need large-capacity systems. In that case, an increase in system capacity should be realized not only by extending conventional multiplex transmission systems described above but also by making effective use of the existing network systems. That is to say, it is important to minimize the extensions.
When concatenation signals, such as STS-192c's (9.953280 Gbps), are transmitted, a new high-speed transmission line and the like dedicated to STS-192c's should not be laid. If there is a transmission line on which the bit rate is limited, these large-capacity signals should be transmitted by making effective use of this transmission line.